The present invention relates to an internal combustion engine that outputs power for driving a vehicle through a power transmission having a gearbox, and more particularly to an catalyst activation controlling apparatus for activating an emission control catalyst, which is located in an exhaust passage of the engine, based on the temperature of the catalyst.
A typical internal combustion engine for driving a vehicle is equipped with an emission control catalyst for purifying toxic from exhaust. When the temperature of the emission control catalyst is lowered, the exhaust cleaning efficiency is also lowered. This causes emission to deteriorate. Japanese Laid-Open Patent Publications No 7-167284 and No. 6-257427 disclose technology for maintaining a high temperature of an emission control catalyst. According to the technology, the gear ratio of an automatic transmission is changed to increase the speed of an engine when an emission control catalyst is not sufficiently heated by exhaust due to a low speed of the engine. The increased engine speed maintains a high temperature of the catalyst.
Incidentally, a “neutral control” for improving the fuel efficiency during idling is known in the art. In the neutral control, when the automatic transmission is shifted to a forward gear and the vehicle is not moving, a forward clutch in an automatic transmission is disengaged or engaging force applied to the forward clutch is decreased so that the clutch slips. This improves the fuel efficiency. When the neutral control is being performed, the exhaust temperature and the exhaust flow rate are lowered. Accordingly, the temperature of an emission control catalyst is lowered. This can lower the exhaust cleaning efficiency and therefore degrade the exhaust emission.
However, when the clutch is disengaged or is slipping, the engine speed cannot be increased by changing the gear ratio. The temperature of the emission control catalyst therefore cannot be prevented from dropping. When the vehicle is not moving, if the engine speed is increased with the intention of increasing the exhaust temperature and the exhaust flow rate, a significantly high engine speed must be maintained since the clutch is disengaged or is slipping and receives no or little load. This adversely affects the durability of the engine. Also, an increase of the engine speed when the vehicle is not moving can disturb the driver.
The similar case also occurs if a driver shifts an automatic transmission into a neutral range or a parking range when a vehicle is not moving. That is, when the engine load is lowered by disengaging the clutch, the exhaust temperature and the exhaust flow rate are lowered, accordingly. This lowers the temperature of the emission control catalyst and thus degrades the exhaust emission. As described above, it is not desirable to prevent deterioration of exhaust emission simply by increasing the engine speed.
Further, if an automatic transmission is shifted to a parking range or a neutral range immediately after an engine is started from a cold state, the engine load is relatively low. Thus, the exhaust temperature and the exhaust flow rate are not sufficiently high. This can prevent the temperature of an emission control catalyst from being increased to a sufficient level for activation or cause the temperature to increase to the level significantly slowly. In these cases, exhaust emission also deteriorates.